1. Field of the Invention
The present invention relates to an optical transmission system in which signal light is transmitted while being amplified with an optical amplifier, and in particular relates to an optical transmission system having a function of automatically eliminating noise occurring due to amplified spontaneous emission light generated in an optical amplifier.
2. Description of the Related Art
With the recent rapid popularization of the Internet and electronic trading and the like, optical transmission technology capable of large-capacity and long-distance transmission has been desired, and wavelength division multiplexing (WDM) transmission technology is currently the subject of much attention. In a WDM transmission system, there are provided optical amplifiers, at intervals of approximately 100km, each of which collectively amplifies a wavelength division multiplexed signal light just as it is, without converting into an electrical signal.
As one of optical amplifiers utilized in WDM transmission systems, a rare-earth element doped fiber amplifier is mainly used, in which a core portion of an optical fiber is doped with rare earth ions such as erbium or the like. When a WDM signal light is amplified using such a rare-earth element doped fiber amplifier, an amplified spontaneous emission light (ASE) is generated in the amplifier. This ASE is superimposed on the signal light, to act as a type of noise, causing a reduction of an optical signal to noise ratio (OSNR). In order to transmit the signal light without an occurrence of error, generally, the OSNR of a given value or greater is required, and the reduction of the OSNR as above described significantly affect the deterioration of system performance.
Currently, the number of channels (number of wavelengths of the signal light) to be wavelength division multiplexed exceeds 170 waves in practical levels, and will further be increased in the near future. Moreover, it is predicted that the bandwidth available to the optical amplification will further be extended in the near future. However, when apparatuses such as the above described optical amplifiers and the like are introduced into a physical circuit, in many cases, the number of wavelengths connected in an initial operation stage is approximately ten waves at most. Therefore, as shown in (B) of FIG. 14 for example, when the number of operational wavelengths is extremely small (or the operational bandwidth is extremely narrow) in relation to the bandwidth of the optical amplifier, the proportion of the signal light to the ASE in the total optical power in the input and output of the optical amplifier becomes extremely small. As a result, the signal light level per channel in the output of the optical amplifier is reduced with respect to a primary target value (design value), causing the deterioration of the OSNR, and the degradation of transmission characteristics.
As shown in a conceptual diagram in FIG. 15, as conventional technology to suppress the deterioration in transmission quality when a small number of wavelengths is operated in such a WDM transmission system, there has been proposed an ASE correction method) of predicting the reduction of signal light level due to ASE by the calculation, and increasing an amplification factor of the amplifier by this amount compared with a normal time, to compensate for the reduction of signal light level (refer to Japanese Unexamined Patent Publication No. 2000-232433).
In the above described conventional technology, in order to determine a reduced amount in the signal level due to ASE by the calculation, information relating to at least the following parameters is necessary:
(1) Input signal light power and noise figure (NF) of the optical amplifier at that time.
(2) Number of wavelengths.
(3) ASE bandwidth of the optical amplifier.
(4) Cumulative ASE in the case where the optical amplifiers are connected in multi-stages.
However, in order to acquire with accuracy the information relating to these parameters, to execute the ASE correction, there are required a mechanism which collects to notify the above information, and means for calculating a required value necessary for the ASE correction based on this acquired information, resulting in the disadvantage of complexity in configuration and control of the system.
Moreover, since the required value necessary for ASE correction is determined by the calculation, an error in correction value is increased depending on the accuracy of each parameter, causing the deterioration of transmission quality. That is to say, in the case where an increment in the amplification factor becomes insufficient due to the error in the correction value, the peak level of the signal light becomes less than a normal (a specified value), and the OSNR is deteriorated abruptly. On the other hand, in the case where the increment in the amplification factor becomes excessive due to the error in the correction value, since the peak level of the signal light becomes greater than the specified value, the pulse waveform comprising information bits is deteriorated due to a non-linear optical effect, such as a four-wave mixing (FWM) effect, a cross phase modulation (XPM) effect or the like, which occurs in the optical fiber of the transmission path.
Furthermore, in the case where the optical amplifiers are connected in multi-stages as shown in FIG. 16 for example, since the ASE generated in the optical amplifier in each stage is accumulated as traveling toward the downstream direction, the reduction of the signal light level due to the ASE is increased in the optical amplifier disposed on the downstream stage, resulting in the significant deterioration in the OSNR. In the above described conventional technology, since the level of the cumulative ASE input to the optical amplifier arranged in the downstream is increased, the increment in the amplification factor for correcting the reduction of the signal light level becomes larger. Therefore, when the number of connection stages (the number of spans) is large, sometimes, it is impossible to realize the required ASE correction in the downstream optical amplifier due to hardware restrictions, and a limit of the ASE correction performance may cause a restriction in the number of spans. Alternatively, in order to increase the number of spans, it becomes necessary to use high-performance optical amplifiers capable of realizing the above described required ASE correction, resulting in an increase of cost.